Ploymer melt extrusion at high rates may lead to a poor product quality due to the appearance of irregularities/defects on the surface of extrudates whenever the wall shear stress exceeds a critical value (Ramamurthy, 1986; Kalika and Denn, 1987). The onset of these defects is a limiting factor for production rates in many industrial operations such as blown film extrusion. These irregularities vary in intensity and form and may be generally classified into surface irregularities or surface melt fracture and gross irregularities or gross melt fracture (Ramamurthy, 1986). A considerable number of studies have addressed this problem over the past three decades and most of them have been critically reviewed by Tordella (1969), White (1973), Petrie and Denn (1976) and Piau et al. (1990).
Although there is still no general agreement on the causes of the phenomenon, it seems that two basic views prevail. According to the first one (Howells and Benbow, 1962; Cogswell, 1977, 1978; Kurtz, 1984; and Mounihan et al., 1990), the polymer extrudate fractures at the die exit due to an abrupt change in boundary conditions that lead to high stretching rates exceeding the melt strength. Kurtz (1984) has also pointed out that prior to the critical stretching, a critical shear stress must be exceeded. In support to this view, a more specific explanation of the polymer fracture has been recently proposed by Tremblay (1991) based on a numerical analysis of the flow at the die exit. According to the second view (Ramamurthy, 1986; and Kalika and Denn 1987), the onset of these extrudate distortions is accompanied by wall slip and failure or adhesion at the polymer-metal interface in the die land.
To characterize the form and intensity of extrudate distortion, various methods have been used in the past. Mainly profilometry techniques have been used to quantitatively describe the extrudate surface roughness (Beaufils et al. 1989, and Tong 1990). Recently, Kurtz (1991) used microphotography to study the factors influencing the sharkskin effect of linear-polyethylenes, a method which is painstaking and time-consuming.